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Figure 8-2. CIDIN Terminology
7/11/96
No. 71
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DATA
LINK
CNTL
FIELD
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X.25
PACKET
HEADER
CIDIN
PACKET
HEADER
CIDIN
TRANSPORT
HlNlER
CIDIN MESSAGE (OR SEGMENT)
DATA
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Annex 10 - Aeronautical Telecommunications Volume ZZZ
8.6.5.2.1 There shall be four protocol levels defined to
control the transfer of messages between CIDIN switching
centres:
shall provide full compatibility with the procedures to be
followed for virtual circuits according to CCI'ST
Recommendation X.25.
8.6.5.2.4 THE: CIDIN PACKET PROTOCOL LEVEL
- the data link protocol level
- the X.25 packet protocol level
- the CIDIN packet protocol level
- the CIDIN transport protocol level
Note I.- The relationship of the terms used is shown in
Figures 8-1 and 8-2.
Note 2.- The details of CIDIN communication ptocedures
and system spec$cations, as implemented in Europe, are
shown in the EUR CIDIN Manual (EUR Doc 005).
8.6.5.2.2 THE DATA LINK PROTOCOL LEVEL
8.6.5.2.2.1 X.25 packets to be transferred between two
CIDIN switching centres or a CIDIN switching centre and a
packet switched data network, shall be formatted into data link
frames.
8.6.5.2.2.2 Each data link frame shall consist of a data
link control field (DLCF), possibly followed by a link dara
field, and shall be terminated by a frame check sequence and
flag (being the second part of the DLCF). If a link data field
is present, the frame shall be denoted as an information frame.
8.6.5.2.2.3 X.25 packets shall be transmitted within the
link data ficld of information frames. Only one packet shall be
contained in the link data field.
8.6.5.2.3 THE X.25 PACKET PROTOCOL LEVEL
8.6.5.2.3.1 Each CIDIN packet to be transferred on
CIDIN circuits between CIDIN switching centres shall be
formatted into one X.25 packet. When a packet switched data
network is used, it shall be permissible to format the CIDIN
packet into more than one X.25 packet.
8.6.5.2.3.2 The integrity of each CIDIN packet shall be
preserved by the X.25 packet protocol by mapping each
CIDW packet onto one complete X.25 packet sequence, as
defined in CCITT Recommendation X.25.
8.6.5.2.3.3 Each X.25 packet shall consist of an X.25
packet header, possibly followed by a user data field (UDF).
8.6.5.2.3.4 The X.25 packet protocol is based on the
application of virtual circuit procedures. A virtual circuit
shall be defined as a logical path between two CIDIN
switching centres, If a packet switched data network is used
to interconnect two CIDIN switching centres, the procedure
8.6.5.2.4.1. Each transport header and the associated
segment shall be preceded by a CIDIN packet header. No
further segmentation of the CIDIN message shall be used
between transport protocol level and CmIN packet protocol
level. Both headers, therefore, shall be used in combination.
Together they shall be rcferred to as the communications
control field (CCF). Together with the message segment they
form CIDIN packets that shall be t~~msrnittefrdo m entry centre
to exit cenLrc(s), when necessary through onc or more relay
centres, as an entity.
8.6.5.2.4.2 CIDIN packets of one CIDW message shall be
relayed independently via predetermined routes through the
network thus allowing alternative routing on a CIDIN packet
basis as necessary.
8.6.5.2.4.3 The CIDIN packet header shall contain
information to enablc relay centres to handle CIDIN packets in
the order of priority, to transmit the CIDIN packets on the
proper outgoing circuit(s) and to duplicate or multiplicate
CDIN packets when required for multiple dissemination
purposes. The information shall be sufficient to apply address
stripping on the exit addresses as well as on the addressee
indicators of messages in AFTN format.
8.6.5.2.5 THE TRANSPORT PROTOCOL LEVEL
8.6.5.2.5.1 Information exchanged over the CDXN shall
be transmitted as CIDW messages.
8.6.5.2.5.2 The length of a CIDXN message shall be
defined by the CIDIN packet sequence number (CPSN). The
maximum permissible lcngth is 215 packets which in effect
results in no practical ljrnitation.
　
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